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From: Julian Macassey
Newsgroups: comp.dcom.telecom
Subject: Re: Radio Interference to Phones
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Date: 2 May 91 07:43:43 GMT
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TELEPHONE INTERFERENCE
(CAUSES AND CURES)
Sooner or later, you are going to become the victim, or
cause of, interference to a telephone or phone system. You may be
lucky and only know someone who is the victim of phone
interference.
The sort of interference we are discussing here is Radio
Frequency Interference (RFI). It is obvious that RFI can be
caused by radio transmitters, and also TV transmitters. The radio
transmitters can be AM broadcast, Two Way, CB, Ham, FM broadcast
and studio links. Less obvious can be garage door openers as well
as anything that generates electric sparks such as electric
motors and transformers that are breaking down.
Any device that would also be registered with the FCC as a
Part 15 Subpart J device is liable to cause RFI. A part 15
subpart J device is any device using a microprocessor and
generating square waves. This means any digital device, including
the onboard computer in the family chariot and the lawn sprinkler
timer. Favorite generators of RFI of this type that are often
overlooked are programmable scanners.
Bear in mind also that many phones today are "feature"
phones which means they too have microprocessors on board and can
interfere with themselves, other phones, or even nearby radios
and TVs. Of all types of phones, the old style desk phones -
rotary types known as the 500 set and the Touch-Tone version 2500
set - are much much less susceptible to interference than modern
electronic phones. Modern electronic phones tend to have
transistors and integrated circuits in them. These semiconductors
act as detectors like the old crystal sets. The amplifiers in
these modern phones then tend to amplify the detected signals.
The result...your good buddy from channel 16 tends to get on the
phone line with you. So a fast simple cure for interference can
often be to drag the old black phone back out of the closet.
There is some hope when it comes to interference. The FCC
has issued a field bulletin on RFI. The document is called
"Bulletin FO-10, Telephone Interference". It is reproduced here
(see box) and copies of it can be obtained from any FCC field
office. Find your nearest FCC office by looking up FCC in the
phone book. The Bell System has considered the problem and
approach it from the viewpoint of interference to regular 500 and
2500 type phone sets. The Bell document concerning RFI is known
as a Bell Systems Practice (BSP). The document is BSP Section
500-150-100.
The FCC document is quite adamant that only authorized
personnel can open phones to modify them against RFI and that
only telephone company personnel can add coils and capacitors to
the telephone protector. Modifications to protectors and internal
modifications to phones are described here for those who are
authorized to do such work.
There are many modifications that can be done by the
consumer. Some of these modifications are very effective. One
modification that can be done quickly and cheaply is the addition
of ferrite cores to handset cords and line cords. Also easy to
add are modular plug-in filters. Requiring more work is the re-
routing of phone wire or the installation of shielded wire. Don't
forget that although opening your phone or fooling with the
protector may be a no-no, you can do whatever you want with the
wire in your home.
Before you tear things apart and spend money, there are
a few things to check. These checks can be done with an
adjustable wrench and a screwdriver. What is checked is anything
on the line itself that could be acting as an antenna or
detector. A dirty connection can work as a diode to detect RF
signals. Go over the internal wiring looking for the following:
1. Corroded connections. Clean and tighten.
2. Loose wire terminations, including set wiring and all jacks
and junction boxes. Tighten any loose screws.
3. Abandoned wire still connected to the line. Remove any wire
not connected to a working phone.
4. Old unused devices still connected to the line. Remove
abandoned phone answering machines, old telephones and bells etc.
If any of the above is what is causing the RFI, until you
fix them, there is little hope that anything else you do will
cure the problem. All of the above fixes are legal for any
subscriber to do without falling foul of the FCC or local
Telephone Company. The exception is of course those with party
lines. Party line subscribers should be wary of adjustments to
wires and telephones and call in the phone company. There are
many ways of wiring party lines and the telephone company will be
familiar with their own equipment. If you mess up the wiring of a
party line phone, it may cease to ring or refuse to dial.
The other legal "do it yourself" fix is attachment of
toroid cores. These cores look like small black doughnuts, by
wrapping wire round a ferrite core a simple effective RF filter
or choke can be made. Ferrite cores are frequency selective, by
the choice of the right material, interference can be effectively
hit on the head.
With modern phones, the most RF sensitive part of the
phone is the electret microphone and its preamplifier circuit. By
application of ferrite cores to the handset cord, there is a fair
chance of easily and cheaply fixing the problem. If you are
hearing radio signals on the phone, there is a way of checking if
the microphone/handset cord is to blame. Dial a partial number to
give you silence, listen for the interfering signals and grab the
handset cord. If the signal changes in volume - gets better or
worse - try a ferrite core.
The best source of ferrite cores in small quantities is
Amidon Associates, 12033 Otsego street, North Hollywood,
California 91607. PHONE: (818) 760-4429. Amidon Associates have
several ferrite "mixes" available. For interference from 500 Khz
to 10 Mhz, i.e. AM broadcast RFI, they recommend their 75
material. For interference from 1 to 30 Mhz they recommend their
73 material. The 73 material should take care of all short wave
Ham and CB interference. For low VHF and channel 2-7 RFI you can
try a ferrite core made with the 43 material which should take
care of RFI between 1 and 70MHz. For best results use the
material that has the lowest cutoff point for your problem. If
the local AM transmitter at 1070 KHz is your problem, use the 75
material, it will give much better attenuation at that frequency
than the 73 material.
For a handset cord, a half inch core is ideal. Wrap
four or five turns of the handset cord through the core and plug
it back into the handset. The cord can be held in place with
black vinyl tape or glue, hot melt glue works well. Experiment
with the positioning of the core. Often having the core by the
handset works best, other times plugging in the cord with the
core by the body of the phone is better. Sometimes a core at each
end of the cord is needed to do the trick. The cores may look
kinda clunky, but if they provide relief easily and cheaply, who
are you to complain.
The numbers for the half inch cores are: FT-50A-75, FT-50A-
73 and FT-50A-43. Yes you guessed it - the last two digits tell
you the material being used. For one and a half inch cores used
with line cords explained below, the numbers are: FT-140-75. The
last two digits being the same as for the half inch cores.
For RFI that you suspect to be entering via the phone
line, wrapping the line cord round a large core can help. Usually
it is best to place the core at the telephone end of the line
cord. Though like all RFI cures, experimentation, otherwise known
as "suck it and see", does a better job than hard and fast rules.
With the large core on the line cord, between six and twenty
turns on the cord should do the trick. A core on each end of the
line cord may help in stubborn cases.
The FCC in their document mention plug-in modular RFI
filters. Despite their breezy assurance, these can be hard to
find locally. Those lucky enough to have an AT&T phone store
handy can buy a device called a "Radio Frequency Filter", it
will cost about $6.00 and is modular. All you have to do is take
it home and plug it in. Those far from an AT&T store can order
one by phone. With credit card in hand, dial (800) 555-8111 and
ask for a Radio Frequency Filter.
For authorized phone repair stations, telephone
personnel and those willing to risk "Open circuit surgery", there
are several solutions. Using ferrite cores, twenty turns or so of
scrap 24 Gauge telephone wire can be wrapped round a half inch
ferrite core. Use two cores, one for Tip and one for Ring and
place them inside the phone. The same kind of cores and windings
can also be used, inside the phone, on the transmitter
(microphone) leads.
For those really handy with a smoking soldering iron
there are some more fixes to try. For phones using electret
microphones, some well placed capacitors may do the trick. Try a
0.01 uF (10 Nf) across the electret element. If that doesn't
work try the same value of cap across the hot side of the
element to the "ground" of the pc board. Regular phones
with carbon transmitters can be helped with a 0.01 or 0.1 uF
capacitor across the element. Solder the capacitor across the
contact fingers in the handset, not across the element, so if
the transmitter is changed, the RF proofing will stay with
the phone. Also, inside the phone, a 0.1 uF (100 Nf) 250V
capacitor across Tip and Ring can be helpful. The type of
capacitor to use is a Ceramic or Mylar.
For those with access to AT&T parts or wishing to help
the local phone company, there are a couple of bits of helpful
hardware mentioned in Bell Systems Practice 500-150-100. First
there is a coil that should be spliced into the phone line. It is
called a 1542A inductor. It should be spliced into the line as
near as the offending telephone set as possible. This means put
it right before the modular jack. It has six terminals, two for
Tip, two for Ring and two for a ground, should the phone still
need a ground (yellow wire) for the ringer or party line. The
ground terminals are not in any way connected to the coil, so
bringing a ground to the inductor, unless needed in the phone,
will not help cure any RFI.
The Bell document also mentions a capacitor, designated
a 40BA capacitor. It is actually four capacitors (see Fig 1) and
the intent is to place a capacitor between each leg of the phone
line and ground. The 40BA is usually installed at the telephone
protector. There is always a good ground available at the
protector, often a heavy gauge solid, solid gray jacketed wire.
Those telephone personnel who do not have access to a 40BA
capacitor should find that a couple of 0.1 uF 250V Mylar
capacitors will work just as well (see Fig 2). To install the
40BA or 0.1uF capacitors, find the protector. The protector is
usually outside the building in a wall mounted small box, in the
basement or in a closet for businesses and apartment buildings.
If the phone line comes in on overhead cable, the protector will
be in the first box the cable goes to after entering the
premises.
That should be some help in beating the problem. Don't
forget that some types of phones are more sensitive than others.
Some cases may be so severe that nothing helps. AT&T no longer
have RFI proofed phones available, although an old style desk
phone with some capacitors added will be pretty immune to RFI.
Alas AT&T no longer makes old style 500 and 2500 desk sets,
although they sell reconditioned ones. Several manufacturers such
as ITT, Comdial, and Northern Telecom still make old style
phones.
END
Julian Macassey, N6ARE
742 1/2 North Hayworth Avenue
Hollywood
California 90046-7142
PHONE: (213) 653-4495
This article appeared on Page 56 of the Feb 1988 edition of
Popular Communications Magazine.
--
Julian Macassey, n6are julian@bongo.info.com ucla-an!denwa!bongo!julian
742 1/2 North Hayworth Avenue Hollywood CA 90046-7142 voice (213) 653-4495
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From: Julian Macassey
Newsgroups: comp.dcom.telecom
Subject: Re: Radio Interference to Phones
Message-Id: <356@bongo.UUCP>
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The folowing is a "Bell Systems Practice" on telephone RFI.
The document is complete apart from two photographs of the 1542A
inductor and the 40BA capacitor. Also missing is the schematic
and and diagram of the 425J schematic. The values of the 40BA
capacitor are shown, normally this capacitor would be a 250 Volt
Mylar type. There is no value for the 1542A inductor.
BELL SYSTEM PRACTICES SECTION 500-150-100
AT&TCo Standard Issue 7, January 1974
RADIO SIGNAL SUPPRESSION
FOR TELEPHONE SETS
1. GENERAL
1.01 This section contains information on the reduction of radio
interference in telephone sets.
1.02 This section is reissued to:
o Add information on single slot coin telephones
o Show 241A amplifier replaced by 241B amplifier.
1.03 Radio interference frequently occurs where a radio station
is located near telephone facilities. Generally the radio signal
is picked up by the wire which acts as an antenna, and
demodulated (changed to audio frequency) by nonlinear components,
such as varistors, transistors and diodes in the telephone set.
HOWEVER DEMODULATION CAN ALSO RESULT FROM CORRODED CONNECTIONS
AND TERMINATIONS.
1.04 Before ordering a telephone set modified for radio signal
suppression, check for the following:
o Corroded connections (inside and outside plant)
o Loose wire terminations (including set wiring)
o Abandoned drop wire still connected to line
o Inside wire connected but not used
o Foreign attachments
Installing a modified telephone set on the customers
premises will not be effective if demodulation is caused by
any of the above conditions.
1.05 Telephone set components that may act as demodulators are:
o Speech equalization varistors in networks
o Transistors in amplifiers of some networks and handsets
o Varistors and transistors in TOUCH-TONE dials
o Diodes in polarity guards
o Click suppression varistors across receiver units
o Carbon transmitters
1.06 With the introduction of the 425J and 4010E networks, the
cause of radio signal demodulation in networks has effectively
been minimized. In these networks the speech equalization
varistors have been replaced by resistors and a strapping option
that provides speech equalization.
1.07 All telephone sets, excluding coin, will be available from
the local distributing house modified for radio signal
suppression. Modification will include changing to new type
network, where applicable, and placing bypass capacitors across
all other components that may act as demodulators as listed in
1.05.
R BEFORE ORDERING A MODIFIED TELEPHONE SET MAKE SURE THAT
E DEMODULATION IS NOT OCCURRING EXTERNAL TO THE TELEPHONE SET
A (See 1.04)
D
1.08 Single slot coin telephones are not modified at the service
center, and where radio frequency is a problem, it is recommended
that a 40BA capacitor and/or a 1542A inductor be added to the
protector and connecting block.
1.09 Where demodulation is attributed to components in a
telephone set, or associated voice coupler, in addition to
changing to a modified set it is recommended that a 40BA
capacitor be placed at the protector and a 1542A inductor at the
connecting block. Refer to section in Division 463 for voice
coupler modifications. The capacitor will bypass, to ground radio
signals picked up by the drop wire while the inductor will tend
to attenuate radio signals picked up by the inside wire.
1.10 Where an adjunct (TOUCH-TONE phone) dial forms a part of
the customers equipment and a modified telephone set is installed
for radio interference reasons, the adjunct dial should also be
replaced by one modified for radio suppression.
1.11 MD telephone sets will not be modified for radio
suppression. Where a telephone set rated MD is encountered, it
should be replaced by an equivalent set in the current series,
modified for radio suppression.
1.12 If possible arrange for operation of the radio station
during the trouble visit in order that the effectiveness of
corrective measures taken may be evaluated immediately.
2. IDENTIFICATION
2.01 Suppression Devices
(a) 425J Network (Fig. 1):
o Eliminates radio interference by having the speech
equalization varistors replaced by resistors and a
strapping option ("H" and "J" leads)
o Can be used for rotary or TOUCH-TONE dial applications
o Contains an equalization network for TOUCH-TONE dials ("X"
Terminal)
(b) 4010E Network (Fig. 2):
o Similar to 425J
o Strapping option consists of "N" and "V" leads
(c) 1542A Inductor (Fig. 3):
o Attenuates RF line current
o Can be substituted for the 42A connecting block
o Removable link between terminals B and Y (current models
only). Required when used as connecting block for telephone
sets with dial lamps or with 30A voice coupler.
(d) 40BA Capacitor (Fig. 4):
o Located at protector to bypass RF signals to ground
2.02 TELEPHONE SET MODIFICATION BY LOCAL DISTRIBUTING HOUSE
(a) Telephone sets, except coin, currently using the 425- or
4010-type networks will be modified as follows:
o Existing network replaced by a 425J or 4010E network,
whichever is applicable
o TOUCH-TONE dial will have bypass capacitors installed
across the varistors and transistors and will also have an
additional (red-slate) lead soldered to the "X" terminal
which in turn connects to the "X" terminal on the network.
o Bypass capacitors installed across all other components of
the telephone set that may act as demodulators, such as
transmitter unit, receiver unit, transistors in amplifiers,
polarity guards, etc.
(b) TRIMLINE telephones will be modified by having bypass
capacitors placed across all the components in the set that
may act as demodulators.
(c) Modified sets will not be recoded but will be identified in
some manner by the local distributing house.
2.03 TOUCH-TONE dials and all G-type handsets will be available
from the local distributing house modified for radio suppression.
These will be used where an adjunct dial must be installed or
where the dial or handset must be replaced for maintenance
reasons.
2.04 Other modified telephone set components include 241B, 242B,
and 277A amplifiers; D-180191 and D-180191 polarity guard
assemblies; and 694A and B subsets. Modified versions of
additional items will be made available in the future when they
are needed.
3. ORDERING GUIDE
o Capacitor, 40BA
o Inductor 1542A-
-49 Gray, -50 Ivory
o Set, Telephone--RF Modified
o Set, Telephone, Hand 220A--RF Modified
o Set, Telephone, Hand 2220B--Modified
o Dial--(TOUCH-TONE dial only) RF Modified
o Set, Hand G--RF Modified
4. INSTALLATION
4.01 TELEPHONE SETS EQUIPPED WITH 425J OR 4010E NETWORKS
(a) Installed in usual manner
(b) For connections see connection section of type set
modified
(c) Sets are shipped with speech equalization option leads
insulated and stored:
(1) For loops greater than 500 ohms leave insulated and
stored.
(2) For loops 500 ohms or less connect "H" or "N"
(Yellow) and "J" or "V" (Orange-Black) leads to
terminals RR and R respectively, on the 425J or 4010E
network.
4.02 Modified TRIMLINE Telephones
(a) Install in usual manner
(b) See Section 502-321-400 for connections
4.03 1542A Inductor (Fig. 5)
(a) Use as connecting block for telephone set cord.
(b) Locate as near as possible to wall-type sets.
4.04 40BA Capacitor (Fig. 6)
(a) Install near and connect to protector with as short as
possible inside wire.
(b) Station wires must connect to the capacitor
(c) Mounted inside when outside protector is used
(d) PROTECTOR GROUND MUST BE UPGRADED TO THE BEST POSSIBLE
GROUND AVAILABLE.
5. MAINTENANCE
5.01 Normal maintenance can be performed on modified telephone
sets. Modified TOUCH-TONE dials and G-type handsets are available
from the local distributing house for maintenance.
END
Fig. 3--1542A Inductor schematic
1542A
INDUCTOR
__________________
RING |R1 R | RED
-------------------|-o--^^^^^^^^^^--o-|----------------
| |
TIP |G1 G | GREEN
-------------------|-o--^^^^^^^^^^--o-|----------------
| |
GROUND | B Y | YELLOW
-------------------|-o-------\------o-|----------------
LINE |__________\_______| SET CORD
\
\
REMOVABLE LINK
Fig. 4--40BA Capacitor Schematic
0.25uF 0.25uF 0.25uF o.25uF
--------| (------| (----o---| (-----| (------|
| | |
| | |
| | |
| | |
| | |
| | |
| | |
| | |
o o o
TIP GROUND RING
--
Julian Macassey, n6are julian@bongo.info.com ucla-an!denwa!bongo!julian
742 1/2 North Hayworth Avenue Hollywood CA 90046-7142 voice (213) 653-4495
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Subject: Re: Radio Interference to Phones
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INFORMATION BULLETIN
Federal Communications Commission
Field Operations Bureau
_________________________________________________________________
Telephone Interference
----------------------
This document has been prepared to assist you in understanding
why interference to your telephone system occurs. Recommended
solutions for you and your authorized telephone service
technician are also provided.
Bulletin FO-10
September 1986
WHY TELEPHONE INTERFERENCE OCCURS
Telephone technology today uses circuitry which, if left
unprotected, will respond to the radio frequency (rf) signals
from nearby radio transmitters such as Amateur, Citizens Band and
AM/FM broadcast stations. When the telephone circuitry responds
to the rf signal you hear the interference on your telephone. The
rf signal can be entering at the telephone instrument or on the
inside or outside wiring.
Cordless telephones are also susceptible to rf signals. Cordless
phones are low-power transmitters using radio frequencies. As
with any radio transmitter, they can receive interference from
other nearby transmitters. Interference can also occur if your
neighbor's cordless phone is using the same radio frequency as
yours. Since the FCC does not offer interference protection to
cordless telephones, you should contact your dealer or
manufacturer for assistance when interference occurs.
RECOMMENDED SOLUTIONS
The installation of a modular filter is suggested as a first
step. Modular filters are available from most telephone stores.
It can be easily installed by you if your telephone has a modular
jack. You may wish to verify if the filter can be returned for a
refund if it does not eliminate the interference.
The effectiveness of filtering may vary according to the type of
telephone you are using. Also, modifying certain types of
telephones, as suggested below, may be impractical or expensive.
When it is, consider changing to another brand or model with
better interference protection.
If you have taken the above steps, and the interference is still
present, the telephone instrument or wiring will probably have to
be filtered. YOU SHOULD NOT INTERNALLY MODIFY THE INSTRUMENT
YOURSELF NOR ATTEMPT FILTERING OUTSIDE PHONE LINES. ONLY
AUTHORIZED SERVICE TECHNICIANS MAY INTERNALLY MODIFY TELEPHONE
INSTRUMENTS. ONLY TELEPHONE COMPANY PERSONNEL MAY FILTER OUTSIDE
TELEPHONE LINES. This rule is designed to ensure that phones
continue to comply with FCC registration standards.
Interference picked up by the inside wiring can be corrected by
installing SHIELDED inside wiring. If you have subscribed to
inside line maintenance, the local telephone company can install
shielded inside wiring for you. If you have NOT subscribed to
inside line maintenance, they can still perform this service for
a fee.
SERVICE TECHNICIAN SECTION
If your customer has tried the modular filter and shielded their
inside wiring and the interference is still present, we recommend
you filter the inside of the telephone instrument.
Interference in older rotary dial phones (without special
features) can usually be resolved by bypassing the carbon
microphone. Install a 0.001 mfd ceramic disc capacitor in the
back of the mouthpiece in the handset. Where possible, solder the
capacitor directly to the microphone contact fingers with the
shortest possible leads.
Phone instruments with special features such as memory, automatic
redial, speakers, push-button dialling, and sound amplification,
contain components which are sensitive to rf signals. Shielding
and bypassing of these components are necessary to isolate the
affected circuit(s). Refer to the design specifications. Bell
System technicians may refer to the Bell Systems Practices Plant
Series Manual Section 500-150-100 for necessary modifications.
Another possible solution would be to install ferrite cores.
These are donut-shaped devices through which the phone cord can
be wound. Ferrite cores are available in various sizes. Use one
with a hole large enough to permit passing the phone cord through
it two or three times.
To install a ferrite core, first disconnect the phone cord
between handset and telephone base. Loop the phone cord through
the core two or three times. Tape or fasten the core as close as
possible to the handset. Reconnect the phone cord. Two ferrite
cores on the handset might be necessary. If so, place one near
the handset and another near the telephone instrument.
If the interference continues after filtering the phones and/or
installing inside shielded wiring, the interference is probably
entering the system through the outside wiring. The local
telephone company service department should be contacted for
assistance. Only telephone company personnel may filter outside
telephone lines. Two devices which may be used for eliminating
outside interference are:
-a 40 BA capacitor installed at the service entrance protector,
and
-a 1542A inductor installed at the connector block.
Your next step would be to have the telephone instrument
filtered. If you own your phone, contact the dealer or
manufacturer for assistance. If you lease your phone, contact the
local phone company's service department. You may wish to share
the Service Technician Information section of this document with
the technician assigned to assist you.
Even though you may be experiencing interference to other home
electronic entertainment equipment, such as televisions, stereos,
or VCRs, the telephone should be filtered. If the rf signal is
entering the telephone system, the interference can only be
eliminated at the point of entry. Filtering or shielding of the
phone instrument also offers future protection to your equipment
from other rf signals.
You may provide to the FCC the name and/or address of the
owner/operator of the radio equipment involved when other
equipment is receiving interference. The FCC will communicate
with that person to determine if they can assist you in
eliminating the interference. FCC office addresses are listed at
the end of this document.
CAUTION: Only authorized service technicians may
internally modify telephone instruments. Only
telephone company personnel may filter outside
telephone lines.
END
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Julian Macassey, n6are julian@bongo.info.com ucla-an!denwa!bongo!julian
742 1/2 North Hayworth Avenue Hollywood CA 90046-7142 voice (213) 653-4495